Internal combustion engine designers have made numerous attempts to extract energy, which would otherwise be lost, from the exhaust gases to improve the power output and fuel efficiency of the engine. A turbocharger is one example of an exhaust energy extraction system which has been used to great success to improve the performance of internal combustion engines. A turbocharger typically includes an exhaust gas-driven turbine mounted on a common shaft with an impeller-type compressor which is designed to supply ambient air under pressure to the intake manifold of the engine.
While turbochargers have been very effective in improving engine performance, the cost and size of conventional turbochargers have limited their universal adoption. Attempts to improve further the performance of turbochargers has led to the development of two stage turbochargers but such units aggrevate even further the problem of excessive size associated with the use of turbochargers.
Accordingly, efforts have been made to design more compact turbocharger units. For example, Zehnder, in U.S. Pat. No. 4,032,262, suggests that the space occupied by a two-stage exhaust-gas driven turbocharger can be reduced by using a common housing for the high pressure and low pressure gas flow passages of the two stage turbocharger. Froeliger, in U.S. Pat. No. 4,196,593, extends the Zehender common housing concept to include orienting the turbocharger shafts of a two stage turbocharger to be mutually perpendicular to reduce the overall size of a turbocharged engine so as to make the turbocharged engine as compact as possible. Curiel et al, in U.S. Pat. No. 4,344,289, discloses arranging the compression ratios of the turbochargers used in a two-stage turbocharger system to provide a compact exhaust-driven turbocharger system.
Another form of exhaust energy extraction system includes a turbocompound system which, normally, includes a turbocharger and a second exhaust gas driven power turbine fluidically connected in series with the turbocharger. The second gas-driven power turbine is located downstream of the turbocharger for receipt of the engine exhaust gases discharged from the turbine of the turbocharger. The power turbine operates to convert energy in the exhaust gases discharged from the turbocharger into mechanical energy which is mechanically coupled to the engine crankshaft to add the mechanical energy output of the power turbine to the engine crankshaft.
Adding a power turbine downstream of the turbocharger turbine imposes a backpressure on the turbocharger turbine which tends to decrease the pressure drop possible for the turbocharger turbine, thereby tending to decrease the potential power output from such turbocharger which is dependent on this pressure drop. Such an effect may require providing a turbocharger of a turbocompound system which is larger in size than a turbocharger in which the pressure drop across the turbocharger turbine is not so decreased. Thus, the turbocharger of a turbocompound system may be so large that overall engine size may offset some of the advantages intended to be realized from the turbocompound system.
Therefore, engines using known turbocompound systems, such as the turbocompound system disclosed by Harp, Jr. in U.S. Pat. No. 4,100,742 may occupy a volume which is still too large to meet demands placed on combustion engines by certain applications where high power high efficiency are required.